It seems clear that platelet production is controlled, in part, by a thrombocytopoiesis-stimulating factor (TSF or thrombopoietin). The studies necessary to determine the site of production of thrombopoietin, its chemical nature, and mechanisms of TSF action have been hampered by the lack of suitable assay procedures. However, recent developments in assay procedures make possible further studies. The objectives of this research proposal are: (1) to continue the development of both a bioassay and an immunoassay for thrombopoietin utilizing TSF obtained from several sources; (2) to develop and characterize antibodies to TSF; (3) to determine the site(s) of formation of thrombopoietin; (4) to purify and identify the hormone, thrombopoietin; (5) to investigate megakaryocytopoiesis by thrombopoietin in vivo and in vitro; and (6) to determine TSF and erythropoietin interactions. These objectives will be pursued by utilizing a variety of physical, chemical, and immunological methods: (1) antiplatelet serum and irradiation will be used to make animals thrombocytopenic for the production of TSF-rich serum; (2) cells grown in culture will be utilized as a method of producing TSF; (3) anti-thrombopoietin sera are to be raised in rabbits; (4) chemical isolation of TSF from various sources will be attempted; (5) organ removal will be performed for determination of site(s) of production of thrombopoietin; (6) measurement of megakaryocytes after injection of potent sources of thrombopoietin will be investigated; (7) spleen colony assays will be performed; and (8) 89Sr, splenectomy, hypoxia, and erythropoietin will be used for the study of TSF and erythropoietin interactions. These studies will provide the necessary information to predict the mode of action of thrombopoietin. In addition, the development of a reproducible quantitative assay procedure for thrombopoietin will make possible studies of TSF action in patients with platelet production disorders.